If it is an inhouse test, it is likely to show positive, but there is a chance it will be false - the FeLV test goes off their antibodies, and with FeLV, they can come into contact (as he is likely to have done), but actually fight it, so you could test him tomorrow and get a positive result, then retest in 12 weeks (recommended timespan as that is how long it can take for them to fight it) and be negative - if they then become negative, they are immune for life. Is there any way he can be isolated pending a Glasgow test?